The present invention relates to methods of acidizing subterranean formations or well bores, and more specifically, to acidizing fluids involving in-situ crosslinked spent acids and crosslinked live acids comprising derivatized xanthan for subterranean acidizing applications. These acidizing fluids may be used in any suitable acidizing treatment to acidize a portion of a subterranean formation or any damage contained therein. The term “damage” as used herein refers to undesirable deposits in a subterranean formation that may reduce its permeability. Scale, skin, and hydrates are contemplated by this term. Also contemplated by this term are geological deposits, such as but not limited to, carbonates located on the pore throats of the sandstone in a subterranean formation.
Acidizing and fracturing treatments using aqueous acidic solutions commonly are carried out in subterranean formations (including those that contain hydrocarbons as well as those that do not) penetrated by well bores to accomplish a number of purposes, one of which is to increase the permeability of the formation. The resultant increase in formation permeability normally results in an increase in the recovery of hydrocarbons from the formation.
Acidizing techniques can be carried out as “matrix acidizing” procedures or as “acid fracturing” procedures. Generally, in acidizing treatments, aqueous acidic solutions are introduced into a subterranean formation under pressure so that the acidic solution flows into the pore spaces of the formation to remove near-well formation damage and other damaging substances. The acidic solution reacts with acid-soluble materials contained in the formation which results in an increase in the size of the pore spaces and an increase in the permeability of the formation. This procedure commonly enhances production by increasing the effective well radius. When performed at pressures above the pressure required to fracture the formation, the procedure is often referred to as acid fracturing. Fracture-acidizing involves the formation of one or more fractures in the formation and the introduction of an aqueous acidizing fluid into the fractures to etch the fractures' faces whereby flow channels are formed when the fractures close. The aqueous acidizing fluid also enlarges the pore spaces in the fracture faces and in the formation. In fracture-acidizing treatments, one or more fractures are produced in the formation and the acidic solution is introduced into the fracture to etch flow channels in the fracture face. The acid also enlarges the pore spaces in the fracture face and in the formation. The use of the term “acidizing” herein refers to both types of acidizing treatments, and more specifically, refers to the general process of introducing an acid down hole to perform a desired function, e.g., to acidize a portion of a subterranean formation or any damage contained therein.
Although acidizing a portion of a subterranean formation can be very beneficial in terms of permeability, conventional acidizing fluids can have significant drawbacks. One major problem associated with conventional acidizing treatment fluids is that deeper penetration into the formation is not usually achievable because, inter alia, the acid may be spent before it can deeply penetrate into the subterranean formation. The rate at which acidizing fluids react with reactive materials in the subterranean formation is a function of various factors including, but not limited to, acid concentration, temperature, fluid velocity, mass transfer, and the type of reactive material encountered. Whatever the rate of reaction of the acidic solution, the solution can be introduced into the formation only a certain distance before it becomes spent. For instance, conventional acidizing fluids, such as those that contain organic acids, hydrochloric acid or a mixture of hydrofluoric and hydrochloric acids, have high acid strength and quickly react with the formation itself, fines and damage nearest the well bore, and do not penetrate the formation to a desirable degree before becoming spent. To achieve optimal results, it is desirable to maintain the acidic solution in a reactive condition for as long a period of time as possible to maximize the degree of penetration so that the permeability enhancement produced by the acidic solution may be increased.
Another problem associated with some current acidizing fluids for subterranean formations is that synthetic polymers or surfactants are utilized to gel acidizing fluids. For instance, to obtain a delayed gel, only a few surfactant gels and polymer fluids will work. Moreover, generally, it is desirable in acidizing for the synthetic polymer to crosslink upon spending so that it may divert the unspent acid into a new portion of the formation. Thus, synthetic polymers are usually the preferred choice for such applications due to the wide range of temperatures at which they function and their ability to tolerate many additives. Note, though, some viscoelastic surfactants are another choice, but may suffer from compatibility issues with additives. Natural biopolymers are thought to be poor choices for crosslinked gel acidizing applications due to the relatively low temperatures they were believed to function at and their relatively poor cross linking ability.
Despite the advantages of using gelling agents in acid treatments, using such gelling agents may be problematic. For example, conventional polymeric gelling agents may leave an undesirable residue in the subterranean formation after use. As a result, potentially costly remedial operations may be required to clean up the surfaces inside the subterranean formation. Foamed treatment fluids and emulsion-based treatment fluids have been employed to minimize residual damage, but increased expense and complexity often result.
Biopolymers such as xanthan would be more desirable to use due to their degradability characteristics. Early experimentation, however, with xanthan yielded less than satisfactory gels. These gels tended to break or undergo syneresis easily, and often looked curdled with a cottage cheese-like consistency. Crosslinking xanthan can be especially difficult and/or impractical because the resultant crosslinked structure has been thought to be unusable and can have strange rheological properties.